KYIV (Reuters) -President Volodymyr Zelenskiy brought home from Turkey on Saturday five former commanders of Ukraine's garrison in Mariupol, a highly symbolic achievement that Russia said violated a prisoner exchange deal engineered last year.

Russia immediately denounced the release. Kremlin spokesman Dmitry Peskov said Ankara had promised under the exchange agreement to keep the men in Turkey and complained Moscow had not been informed.

In honour of the 500th day of the war, Zelenskiy also visited Snake Island, a Black Sea outcrop which Russian forces seized on the day of the invasion and later abandoned.

The five commanders have been lionised in Ukraine after leading a fierce three-month defence of Mariupol from the Azovstal steel plant last year, the biggest city Russia has captured.

"We are returning home from Turkey and bringing our heroes home," said Zelenskiy, who met Turkish President Tayyip Erdogan for talks in Istanbul on Friday.

Thousands of civilians were killed in Mariupol when Russian forces laid the city to waste in the first months of the war. The Ukrainian defenders held out in tunnels and bunkers under the Azovstal plant, until finally ordered by Kyiv to surrender in May last year.

Moscow freed some of them in September in a prisoner swap brokered by Ankara, under terms that required the commanders to remain in Turkey until the end of the war.

Peskov told Russia's RIA news agency: "No one informed us about this. According to the agreements, these ringleaders were to remain on the territory of Turkey until the end of the conflict."

Peskov said the release was a result of heavy pressure from Turkey's NATO allies ahead of next week's summit of the military alliance at which Ukraine hopes to receive a positive sign about its future membership.

In his remarks, Zelenskiy gave no explanation for why the commanders were allowed to return home now. Turkey's Directorate of Communications did not respond to a request for comment.

THANKS TO TURKISH PRESIDENT

In a ceremony later alongside the men in the western city of Lviv, Zelenskiy thanked Erdogan for helping secure their release and pledged to bring home all remaining prisoners.

He said that before the outbreak of war, "many people in the world still did not understand what we are, what you are, what to expect from us and what our heroes are. Now everyone understands."

Many Ukrainians hailed the return of the men.

"Finally! The best news ever. Congratulations to our brothers!" Major Maksym Zhorin, fighting in eastern Ukraine, said on the Telegram messaging app.

Referring to a counter-offensive launched by Ukrainian forces in the past month, Denys Prokopenko, one of the five commanders, told the gathering that his men "will have our word to say in the battles. The most important thing is that Ukraine has seized the strategic initiative and is advancing."

U.S. Secretary of State Antony Blinken marked the 500 days by describing Russia as "the sole obstacle to a just and lasting peace" and promising to back Kyiv "for as long as it takes".

France's foreign ministry said the time frame "must bring Russia to the realisation that it is in an impasse and immediately stop its illegal war of aggression".

The latest U.S. pledge of support included plans to supply widely banned cluster munitions. Defence Minister Oleksiy Reznikov promised the munitions would not be used in Russia.

The general staff of Ukraine's armed forces said Ukrainian forces on Saturday "continued offensive operations" in two sectors in the southeast.

Officials say Ukrainian forces have also taken back areas around the shattered eastern city of Bakhmut -- captured by Russian forces in May after month after months of battles.

(Reporting by Olena Harmash; Editing by Peter Graff, Ron Popeski and Daniel Wallis)